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McFly September 19, 2013 08:01

Non-iterative time advancement

I am running OpenFOAM and Fluent in a transient, laminar case on a large mesh.

Fluent is outperforming OpenFOAM on speed (icoFoam or pisoFoam), possibly because the Fluent case is using non-iterative time stepping.

Is this type of feature implemented in OpenFOAM? I have tried to find out more information however searching doesn't come up with anything. Is this the 'fractional step method'?

Apologies if this is a stupid question. The fact things are named differently across the codes is very confusing to me sometimes.



McFly September 23, 2013 02:15

Perhaps I should pose this question differently.

If Fluent and OpenFOAM are running the same mesh, same convergence criteria, same turbulence model (nil) - why should OpenFOAM run so much slower than Fluent?

I realise this is a simplification, however I'm stumped. This project was to determine which package would be better for the case, and so far Fluent is faster than expected, although I believe that's more to do with my lack of knowledge than the packages themselves...

joegi.geo September 23, 2013 06:39

Try to setup exactly the same method in fluent and openfoam. The NITA in fluent is a coupled method, which is a way much faster than segregated methods, but it uses more memory. As far as i know this method is not implemented in the official release of openfoam.

Try to use the piso method in fluent and openfoam, you should get the same results (or close). Be careful to setup exactly the same parameters, namely, time-step, relaxation-parameters, numerics, bc and ic, solvers (multigrid) and convergence criteria, and run fluent with no gui.

In my personal experience fluent is faster, but not that much.


McFly September 25, 2013 22:11

G'day Joel,

Thanks very much for the information. I will look over the cases again and try to ensure equivalency.

I've not found any examples where NITA has been implemented in OF, official or not, which surprises me as it has been around for years.

However, again, that could just be my lack of knowledge on the topic.

Thanks again,


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